The degree is the highest exponent in a term, the term with the highest exponent is -m^4, so the degree is 4. Terms are separated by operation signs such as +, -, ×, ÷. So the number of terms here are 3.
1 term = Monomial
2 terms = Bionomial
3 terms = Trinomial
4+ terms = Polynomial
So this is a 4th degree trinomial.